An imaging device, or sensor, is a photosensitive electronic component used to convert electromagnetic radiation into an analog electrical signal. This signal is then amplified then digitized using an analog-to-digital converter and finally processed so as to obtain a digital image.
Imaging devices make use of the photoelectric effect whereby incident photons tear electrons from atoms in each active element. An imaging device generally comprises photosites arranged in a matrix, each photosite corresponding to a pixel of an image.
A photosite comprises at least one photosensitive region, especially a photodiode, and a region for reading charge accumulated in the photodiode. Photons captured by a photodiode of a photosite are converted into electron/hole pairs. Charge of the first type, for example holes, created in the photosensitive regions, is stored in the photosite before being read by virtue of an electronic system, while charge of the second type, for example electrons, is drained into n+-type drain regions.
Generally this electronic system, which controls the photodiode, comprises, especially when the photodiode is a fully depleted photodiode, a transfer transistor permitting the transfer of charge stored in the photodiode to a charge reading region. This charge reading region forms a sensing node to which conventional control electronics are connected, especially comprising a read transistor.
A photodiode operates according to a cycle comprising at least an integration step, a sensing step and a reset step. The integration step corresponds to the photogeneration and accumulation of charge during exposure of the photodiode. The sensing step corresponds to the generation of a signal depending on the amount of photogenerated charge accumulated in the photodiode. The reset step corresponds to the removal of the photogenerated charge.
To produce a global shutter the photosites of an imaging device are produced in the conventional way, four transistors being associated with a photodiode; however, the electronic system of each photosite is made more complex, especially by the addition of a fifth transistor.
Moreover, it is known from the article by M. Funaki et al. entitled “New Global Shutter CMOS Imager with 2 Transistors per Pixel” published in “2008 Symposium on VLSI Technology Digest of Technical Papers” to produce photosites using only two transistors, the first transistor controlling charge transfer and the second transistor controlling reading of the signal. The signal delivered by this read transistor is modulated by the charge transferred and stored in its own body (charge modulation device).
However, in such a photosite, the charge is surface transferred, and the read electronics, especially comprising a charge transfer transistor and means for reading charge, is placed mainly on the surface of the photosite occupying most of its useful area and consequently reducing the area of the photosensitive region of the photodiode within the photosite.